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Abstract
Cadmium (Cd2+) ion is one of the most crucial industrial pollutants that cause serious harm to the human body. We proposed and experimentally demonstrated a highly sensitive Cd2+ sensor based on hydrogel coated excessively tilted fiber grating. The hydrogel with the functional monomer of the allyl thiourea can specifically bind to Cd2+, and hence forming a complex. The grating excites high order cladding modes, and ensures a sufficient interaction between the light and hydrogel binding to Cd2+, providing highly sensitive monitoring. The results show that the sensor can detect 0-160 pM Cd2+ in aqueous solution. The maximum sensitivity is 10600 nm/µM, and the minimum detection concentration is 20 pM (about 0.004 ppb), which is much less than that of the international standard (3 ppb). The proposed sensor exhibits high sensitivity, ultra-low detection limit, specificity, and a compact structure, offering potential as a tool for Cd2+ detection in aqueous solution.
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1. Introduction
Cadmium ion (Cd2+), as one of toxic heavy metal ions in the environment, is widely used in plastics, batteries, pigments, and other industrial fields. It is estimated that annually around 30,000 tons of cadmium are released into the environment, of which 13,000 tons result from human activity [1]. Due to the non-biodegradable nature, cadmium can continuously accumulate in the organism and eventually reach the human body through the food chain. Unfortunately, Cd2+ metabolizes very slowly in the human body and is difficult to be eliminated by excretion. The average dose of cadmium poisoning is 100 mg, and long-term intake of cadmium-containing foods will cause damage to human kidneys, thereby inducing a series of serious diseases. The WHO sets the suggested value of Cd2+ in drinking water and industrial wastewater below 3 ppb [2]. Recorded by China national standards, the maximum allowable mass concentration of Cd2+ in industrial wastewater and urban sewage is 0.1 mg/L [3]. Therefore, it is quite necessary and promising to develop cost-effective chemical sensors exhibiting high selectivity and sensitivity, low limit of detection (LOD) and fast response, to be able to detect cadmium ions in drinking or tap water.
Many analytical techniques have been used to detect Cd2+ in ambient safety monitoring, including fluorescence spectroscopy [4], atomic absorption spectrometry [5], plasma optical emission spectrometry [6]. Generally, these methods are stable, effective, and highly sensitive. However, these technologies rely on the support of large and precise laboratory-based instruments and skilled operators. In additional, the sample pretreatments are complicated, time-consuming, and depends on manual field sampling, which cannot achieve real-time online detection. Alternatively, the trace amounts of Cd2+ in water samples can be determined by using the electrochemical method [7] and fluorescent and colorimetric detection [8]. These techniques can be adapted for in situ portable sensor deployment and real-time monitoring of the environment, but provide a relatively high LOD.
Optical fiber sensors possess the advantages of a compact structure, anti-electromagnetic interference, high sensitivity, fast response, real-time remote monitoring, and so on. In the field of Cd2+ ions detection, some optical fiber sensors have been reported including surface plasmon resonance (SPR), optical fiber interferometers, and the fiber couplers. Caucheteur et al. [9] reported a gold-coated tilted fiber Bragg gratings to excite the surface plasmon resonance (SPR) and hence determine the concentration of Cd2+ in solution with a LOD close to 1 ppb. Sharma and Gupta [10] fabricated a fiber-optic SPR sensor based on coating silver and SnO2 nanoparticles over silica optical fiber, which detected the Cd2+ concentration with a maximum sensitivity of 23.7 nm/µM, and a LOD of 14 nM. Feng et al. [11] proposed a reflective fiber-optic SPR sensor with sliver/PVA/TiO2 sensing membrane. The results showed that the sensor can detect 0-1 µM Cd2+ in aqueous solution. The maximum sensitivity of Cd2+ is 315.2 nm/µM, and the LOD is about 0.76 ppb. Feng et al. [12] also proposed an inline fiber-optic Mach–Zehnder interferometer coated with SnO2-MoS2 bilayer film, which gives a sensitivity of 30.258 pm/µM. In 2019, Huang et al. [13] proposed a Cd2+ ions sensor by using an optical fiber modal interferometer based on a fused taper. The sensitivity is 5.126 × 104 nm/M, and the minimum detection limit is 4 × 10−7 M. In 2022, Zhou and Huang et al. [14] detected the Cd2+ ions based on an air-hole-assisted multicore micro-structured optical fiber. A thin hydrogel film was coated on the exposed core. The sensitivity can reach 7.443 × 109 nm/(M) and the experimental LOD is 6.0 × 10−12 M. Shen et al. [15] proposed trace detection of Cd2+ ions based on α-Fe2O3@MoS2 functionalized optical microfiber coupler. The results showed that the maximum detection sensitivity of Cd2+ is 9758.92 nm/µM, and the minimum detection concentration is 0.0001 µM (about 0.0182 ppb).
In this paper, we proposed a highly sensitive Cd2+ ions sensor based on an excessively tilted fiber grating (Ex-TFG) coated with hydrogel film. The Ex-TFG couples guided light out of the fiber core and into the cladding. The excited high order cladding mode is highly sensitive to the surrounding refractive index (SRI). A thin hydrogel film as the sensing membrane was coated on the surface of Ex-TFG by using the dip coating method. The transmission spectral of the Ex-TFG and the response to the SRI were studied both experimentally and theoretically. The Cd2+ sensing performance was investigated in the concentration range of 0-160 pM. The results indicated a sensitivity of 10600 nm/µM, and an experimental LOD of 20 pM.
2. Sensing principle
2.1 Theoretical model of Ex-TFG
Compared the traditional non-tilted fiber gratings, the tilted fiber gratings have stronger and more efficient cladding mode couplings and polarization dependency. In detail, for tilted fiber gratings, the fundamental mode with azimuthal order m = 1, is allowed to be coupled to the cladding modes with azimuthal order m = 0, 1, 2, 3 [16]. Furthermore, the coupling is polarization dependent even if the gratings are written in circular fibers. For Ex-TFG, the large tilted angle allows the cladding mode couplings with large radial order n > 20 while the long period fiber grating only excites the cladding mode with radial order n < 10. The Ex-TFG with a distinctive core-to-cladding mode coupling provides an accessible evanescent field to enhance light-matter interaction. The Ex-TFG have been developed for relatively humidity sensing [17], glucose sensing [18], label-free immunosensor [19], and Pb2+ sensing [20].
Full vector complex coupled mode theory (FV-CCMT) [21] was employed to fully understand the mode coupling and polarization dependency in Ex-TFG. In the FV-CCMT, the fundamental core mode is coupled to the TM/TE cladding modes and the EH/HE cladding modes. Considering the cladding mode with large radial order, the transverse field of the HE modes eventually becomes close to purely azimuthally polarized (quasi-TE) and that of the EH modes becomes strongly radially polarized (quasi-TM) [22], and the typical intensity filed of the TE, TM, and HE/EH modes can be found in numerical investigation of the mode coupling analyses [23].
Figure 1(a) shows the typical schematic of the Ex-TFG with a tilted angle θ. The refractive index of the fiber core is periodically modulated after UV irradiation. Here, Λ and Λg are the axial period and normal period of the grating, respectively. The relationship between Λ and Λg is Λg = Λcosθ. In a cylindrical coordinate system, the index variation Δn (z, r, ϕ) in the fiber core is given by
Fig. 1. (a) Schematic of the Ex-TFG. (b) Optical microscope image of the tilted fringes in the Ex-TFG. (c) Micrograph of the sensor after coating hydrogel film.
In the Ex-TFG, the phase matching condition is given by
where λ is the resonance wavelength; $n_{eff,11}^{co}$ is the effective index of the fundamental core mode HE11; $n_{eff,mn}^{cl}$ is the effective index of cladding mode with azimuthal order m and radial order n.2.2 Chemical mechanism of the sensing membrane
The sensing principle of hydrogel for detecting the metal ion is to form coordination between metal ion and its corresponding ligand, and the Cd2+ selectivity relies on that functional groups that can specifically bind to Cd2+ [24]. S atom in thiourea can specifically bind to Cd2+ due to its high electron cloud density, and the empty orbitals of Cd2 + perfectly match it, forming a complex. Therefore, allyl thiourea was used as the functional monomer in hydrogel preparation [25]. Allyl thiourea and Cd2+ belong to soft base and acid, respectively. The functional group (C=S) in allyl thiourea has inherent selectivity, hence the coordinate covalent bond reaction between allyl thiourea and Cd2+ can generate stable complexes according to Lewis theory of acid and bases [26]. Besides Cd2+, K+, Na+, Ca2+, Mg2+, Mn2 + and Fe3+ are hard acid ions, and Cu2+, Zn2+, Pb2 + and Ni2+ are boundary acids. Additionally, the size of the void between the effective functional groups in the hydrogel is close to the relevant parameters of Cd2+, which makes it easier for Cd2 + to enter than other large size ions, and harder to come out than other smaller size ions. Therefore, the Cd2+ can selectively bind well with the hydrogel.
When the Ex-TFG modified with the sensing layer is immersed in the Cd2 + solution, the thiourea group and Cd2+ form a “S-Cd-S” cross-linking structure due to the large binding constant, which increases the cross-linking degree of the hydrogel. The coordination between Cd2+ and the thiourea group will reduce the amount of total charges, thus reducing the osmotic pressure, leading to the volume shrinkage of the hydrogel. On the contrary, the hydrogel will swell when absorbing water. Finally, the hydrogel with Cd2+ will reach an equilibrium state. Different concentrations of Cd2 + solutions lead to different equilibrium states, which is reflected in the change of the refractive index of the hydrogel. The light coupled from the Ex-TFG interacts with the hydrogel, leading to the wavelength shift of the resonant dips and achieving the detection of the concentration of Cd2+ in the solution.
3. Materials and methods
3.1 Hydrogel preparation
Acrylamide (AM, 99%), N, N’-Methylene-bisacrylamide (bis-AM), 1-allyl-2-thiourea (ATU), 2,2- α,α-diethoxyacetophenone (DEAP), 3-Aminopropyltriethoxysilane (APTES) and Ethanol absolute were purchased from Shanghai Aladdin. All materials were of analytical pure. In order to facilitate the hydrogel coating, we pre-polymerized the solution to increase its viscosity. We take AM (32.4 mmol) and ATU (15 mmol) as monomers, bis-AM (2.5 mmol) as crosslinker and DEAP (10 µL) as photoinitiator. All them were mixed with 1.5 ml deionized water in a beaker, and sonicated for 30 min to obtain a clarified and transparent solution. In order to prevent strong oxygen inhibition during the polymerization reaction, N2 was continuously added in the solution for 10 minutes. At this time, a small amount of solute may precipitate again. The solution was sonicated again for 1 minute to return clarity and transparency. To verify the success of the solution, the appropriate solution was placed in a sample tube and exposed under a UV light. The light was turned off immediately as long as the viscosity of the solute was increased. The solution was stored in dark environment at room temperature.
Figure 2(a) and (b) shows the SEM morphology of the hydrogel before and after binding to Cd2+. It can be seen that the smooth surface of the hydrogel become rough after combining with Cd2+. Besides the SEM morphology, we also used Energy Dispersive Spectrometer (EDS) to qualitatively analyse the elements contained in the sensing material. The basic principle is as follows: The sample is bombarded with a finely focused electron beam, and exciting the characteristic X-rays of the elements contained in the sample. Different elements have different X-ray characteristic wavelengths and energies. The types of elements contained in the microregion can be qualitatively determined by identifying the characteristic wavelengths or characteristic energies. Figure 2(c)-(f) show the elemental mapping patterns. The colors represent the types of the elements, and the density qualitatively represents the amount of elements in the microregion. It is found that the sensing material contains a large amount of C, N, and O elements. Figure 2(f) distinctly testify the existence of Cd2+, and Cd is successfully bound and uniformly distributed in the hydrogel. To future reveal the reaction between the Cd2+ and hydrogel, we compared the FT-IR spectra of hydrogel before and after binding to Cd2+, as shown in Fig. 2(g). For the blank hydrogel, two sharp and narrow absorption peaks are found at around 1650 cm−1 and 984 cm−1, which result from the vibration of the C = S bond and the C–S bond, respectively. After binding to Cd2+, these two peaks have undergone clear degradation as shown in partially enlarged images. The S atom is originally only subjected to forces from C atom, but now it is subjected to a dual force from both C and Cd atoms when the C-S and C = S bond connects with Cd atom. In other words, the force of C-S and C = S bond is partially cancelled by S-Cd bond. Therefore, the peaks originated from C = S and C-S bond tends to become degradation. These test results support the sensing membrane bond with Cd2+ ion successfully.
Fig. 2. Characterization of the sensing membrane. SEM images of the hydrogel (a) before and (b) after binding to Cd2+. (c)-(f) are the corresponding elemental mapping patterns for Pt, N, O and Cd. (g) FT-IR spectrum of sensing membrane.
3.2 Fabrication of Cd2+ sensor
The tilted fiber grating was fabricated by using a 244 nm UV beam to scan a tilted amplitude mask over a commercial single mode fiber (Corning SM 28). The laser power is 130 mW, the scanning speed is 0.03 mm/s, and the grating length is around 7 mm. The custom designed amplitude mask has a period of 6.6 µm. The amplitude mask was rotated to generate a tilted UV beam. Due to the cylindrical geometry of fiber, the tilt angle of UV fringe outside of fiber is different to that inside of fiber [27]. In the experiment, the amplitude mask tilted at ∼80° would produce a highly tilted fringes at ∼83° in the fiber core, giving an axial period of 38 µm. Figure 1(b) shows the sideview micrograph of the 83°-TFG, which displays that the whole fiber core is modulated by the tilted fringes.
Before coating the hydrogel layer, the Ex-TFG was pre-treated as follows. The Ex-TFG was put into dilute salt water for soaking and cleaning. It was taken out and rinsed with deionized water, then it was put into the oven and dried at 60 °C. In order to make the fiber and hydrogel bond more closely, the surface of the fiber was first coated a layer of APTES. The specific steps are to add APTES to anhydrous ethanol at a concentration of 1%, dip an appropriate amount of APTES onto the fiber surface with a medical cotton swab, and dry the fiber at 60 °C.
The hydrogel layer was coated on the surface of the Ex-TFG by using the commercially available dip coating equipment. The hydrogel solution was placed in a cuvette. The Ex-TFG was clamped with a dip-coater fixture and lifted out of the cuvette. Based on our previous experience in preparing gel film [28] and the viscosity of the hydrogel in this experiment, we set the lifting speed to 700 mm/min. The fiber was exposed under a UV light for 10 s. The hydrogel film needs a certain thickness to provide enough absorption sites and also make the sensing material not easy to fall off. However, too thick film is also not conducive to allowing the Cd2+ to enter the inner layer of the sensing film, affecting the interaction between light and matter and also increasing the response time. Here, the operation was repeated for three times. After curing, the fiber was put into an oven at 50°C for 24 hours. Finally, a layer of sensing material was uniformly assembled on the surface of the fiber, as shown in Fig. 1(c).
4. Experimental setup and results
Figure 3 shows the experimental setup. the Ex-TFG was connected with a broadband light source (SC-5-FC, YSL Photonics, China) with light spectrum range from 470 nm to far infrared, and the transmission spectrum was measured by an optical spectrum analyzer (OSA, AQ6370C, Yokogawa, Japan) with a wavelength range from 600 to 1750 nm and wavelength resolution 0.02 nm. In order to investigate the polarization dependence of the Ex-TFG, an in-fiber polarizer and a 3-paddle manual fiber polarization controller ere used to obtain p/s- polarized light manually, and hence excites the mode coupling from fundamental core mode to (quasi-)TM/TE cladding modes. Here, p-polarization is in parallel to the grating tilt plane, while s-polarization is perpendicular to the grating tilt plane. The (quasi-) TM/TE modes are radially and azimuthally polarized, respectively.
Fig. 3. Experimental setup for the Cd2+ sensing based on Ex-TFG. PC: polarization controller, OSA: optical spectrum analyzer.
4.1 Characterization of the Ex-TFG
Figure 4(a) shows the experimental transmission spectra of Ex-TFG. The multiple resonant dips indicate that the fundamental core mode is coupled to a series cladding modes according to phase match condition in Eq. (2). The resonant dips always appear in pairs, generated from the coupling to the two sets of cladding modes of orthogonal polarization statuses. In the wavelength range from 1270 to 1700 nm, near 7 pairs of dual peaks are observed.
In the numerical calculation, the parameters of the Ex-TFG defined in Fig. 1(a) are given as follows: single mode fiber with core and cladding radius rco = 4.5 µm, rcl = 62.5 µm; the fiber grating with Λg = 6.6 µm, θ = 83°, L = 7 mm, σ = 2γ = 8.0 × 10−4. The transmission spectrum covers a large wavelength band from 1270 to 1700 nm, and the material dispersion needs to be considered. The refractive indices of the pure silica cladding and the germanium-doped core are calculated using Sellmeier equation and Fleming’s formula [29]. In the simulation, the dopant concentration of GeO2 is 4.3%.
Figure 4(b) shows the numerical transmission spectra of Ex-TFG. In the previous work, only 5 pairs of dual dips were predicted by solving the eigenvalue equations for TM0n and TE0n of a circular waveguide [30]. By using the FV-CCMT method, the calculated spectra also have near 7 pairs of dual dips, and the resonant dips of the calculated spectra located near those of the experimental spectra. The FV-CCMT method includes the vector modes with azimuthal order m = 1, 2, 3 besides the TM0n and TE0n modes with azimuthal order m = 0. Therefore, the simulation spectrum is almost completely consistent with the experimental spectrum.
4.2 Refractive index sensing
We firstly examined the SRI response of the bare Ex-TFG by using a series of refractive index liquids. Seven glycerol solution samples with different weight concentrations (0%, 10%, 20%, 30%, 40%, 50%, and 60%) were prepared for SRI measurement. The corresponding refractive indexes (at 25°C) are 1.3330, 1.345, 1.358, 1.371, 1.384, 1.398, and 1.413, which were measured by an Abbe refractometer. Due to the similar response for both TM and TE dips, only the spectral evolution of the TM branch was presented in Fig. 5(a). Figure 5(b) shows the wavelength shifts of both TM and TE dips. The resonant wavelengths demonstrate the exponential red-shift trends with the increasing SRI. According to our theoretical analysis, the dual dip feature of Ex-TFG is caused by the different mode index of TE and TM cladding mode. When the SRI is increasing, the mode indexes of TM and TE cladding mode are changing, and the index difference between TE and TM mode is decreasing. Therefore, the separation gap between TM and TE dips will decrease when the SRI is increasing. As depicted in Fig. 4, the dip separation gap is around 5.1 nm in the air, and the separation gap is decreased to 2.2 nm in the water as show in Fig. 5(b). At the SRI of 1.413, two resonant dips almost overlapped together, and the separation gap is only 0.13 nm. When the SRI is approaching to the refractive index of the cladding, light guided in the cladding is more easily to leak into the environment, indicating a trend of increasing the refractive index sensitivity. When the SRI is higher than the guided cladding mode index, the cladding then can no longer be supporting any cladding modes, and the resonant dips gradually disappeared. Compared with wavelength shift of the TM and TE resonant dips, the total wavelength shift is 26.3 and 24.1 nm when the SRI is increased from 1.333 to 1.413. The SRI sensitivities of TM and TE modes are ∼288.6 nm/RIU and ∼260.6 nm/RIU near the SRI of 1.384, respectively. In the experiment described below, with the consideration of relatively higher sensitivity of TM resonant dip, only TM resonant dip was employed for the Cd2+ ions detection.
Fig. 5. Refractive index sensing of bare Ex-TFG. (a) Evolution of the transmission spectra of the TM branch. (b) Resonant wavelength as a function of the refractive indexes.
4.3 Cd2+ sensing
The Cd2 + standard solution was purchased from Beijing Tanmo Quality Inspection Technology Co., Ltd. Its concentration is 1000 mg/L. We took an appropriate amount and diluted it with high-purity water to obtain 1µM Cd2 + mother liquid. The mother liquid was continuously diluted with a 100 ml volumetric flask to obtain Cd2+ ion solutions with different concentrations of 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, and 240 pM.
When the Ex-TFG sensor was immersed into Cd2+ water solution, the thiourea group in the sensing layer material captured the Cd2+ ions in the solution. Due to the formation of “S-Cd-S” cross-linking structure and the reduction of osmotic pressure, the volume of the hydrogel shrank, and hence the transmission spectrum exhibited “red” shift due to the increase in the refractive index of the sensing membrane. Figure 6(a) show the resonant spectra of the Ex-TFG at different Cd2+ concentration from 0 (pure water) to 160 pM. It can be observed that the resonant dips have red shift with the increase in the Cd2+ concentration. Initially, the resonant dip located at 1638.24 nm, and shifted by 1.68 nm when the Cd2+ concentration was increased to 160 pM. According to Fig. 5(b), the refractive index of the hydrogel coating is roughly estimated to be around 1.4. After absorbing Cd2+, the refractive index of the hydrogel is increased by 0.003, giving a red shift of 1.68 nm. Figure 6(b) shows the wavelength shift of the resonant dip as a function of the Cd2+ concentration. It is clearly found that there is a nearly line response below the concentration of 120 pM. The sensitivity is around 10600 nm/µM. The wavelength shift for 20 pM Cd2+ is around 0.072 nm which is larger than the wavelength resolution of 0.02 nm. Therefore, it can be conducted that the experimental LOD is 20 pM (about 0.004 ppb) which is much less than that (3 ppb) of the international of the WHO.
Fig. 6. Cd2+ ions sensing of hydrogel coated Ex-TFG. (a) Evolution of the transmission spectra when increasing the Cd2+ concentration. (b) Resonant wavelength as a function of the Cd2+ concentration.
Selectivity is an important parameter for the detection of heavy metal ions in the practical application. Therefore, the comparison group mainly consists of heavy metal ions. In the experiment, the sensor was immersed in solutions containing Cd2+, Pb2+, Cu2+, Zn2+, Mn2+, and Na+, and Hg2+ with concentrations of 120 pM, individually. The experimental result was shown in the Fig. 7. Compared to the dip wavelength shift of Cd2+, the spectral wavelength shift corresponding to the comparison group was relatively small. The results indicate that the sensor has good selectivity for Cd2+. This selectivity is determined by the strong covalent bond reaction between allyl thiourea and Cd2+ as discussed in the sensing principle.
Fig. 7. Selectivity of the Cd2+ sensor based on hydrogel coated Ex-TFG when the ion concentration is 120 pM.
4.4 Discussion
We compared the sensing performance of the reported Cd2+ detection based on optical fiber sensors as shown in Table 1. According to optical mechanism, three optical fiber sensors have been widely employed for Cd2+ sensing, including the SPR, optical interference, and the mode coupling the optical fiber. For SPR method, a thin Au or Ag film was usually coated on the surface the multimode fiber for exciting the surface plasmon wave. The reflective SPR design makes sensor work as a probe for detection, and allows light to pass through the sensing area twice, and hence lowered the LOD to 6 nM (around 0.76 ppb) [11]. For interference method, special fiber structures were designed to excite the cladding modes, thereby interfering with the core mode. The reported special fiber structures include the photonic-crystal fiber [12], fused tapering fiber [13], and air-hole-assisted microstructured fiber [14]. In the modal interferometer, low order cladding mode is usually excited and the effective mode index is difficult to be changed by the absorption of Cd2+ ions. Therefore, the sensitivity is limited due to the small change in the interference phase. In the air-hole-assisted microstructured optical fiber, the thin silica core (∼1 µm) will not be sufficient to bind the light effectively, and high order mode is excited. The exposed side provides a microfluidic channel, which ensures the strong interaction of analyte liquids with light [14]. Therefore, the sensitivity is improved to 7443 nm/µM, and the experimental LOD is 6 pM. For the mode coupling method, desired modes can be excited by using the microfiber couplers and gratings. The microfiber couplers with lumbar cone diameter of 10.5 µm can excite the super odd and even modes, and the sensitivity reached to 9758 nm/µM [15]. In our work, the Ex-TFG excited the ultra-high order cladding modes (n = ∼ 30), providing a strong interaction between the light and the Cd2+ solution. The sensitivity is as high as 10600 nm/µM. The experimental LOD (20 pM) is comparable with those based on air-hole-assisted micro-structured fiber and microfiber coupler. In contrast with the side exposed micro-structured fiber and the microfiber, the Ex-TFG was fabricated in the standard single mode fiber, providing more robust sensing units.
Table 1. Comparison of the Cd2+ ions sensing performance of the typical optical fiber sensors
Besides the optical principle and optical fiber structure, the Cd2+ sensing performance also relies on the sensing material. Metal oxides and metal sulfide are widely used for Cd2+ sensing. The bonding of Cd2+ by the OH groups plays a key role by using the metal oxides [11]. For the metal sulfide, the Cd2+ is also strongly chelated by surface functional groups -S [32]. However, the previous experimental results have verified that the individual metal oxides or metal sulfide gave a relatively low sensitivity. The mixed membrane or bilayer membrane have to be used for improving the sensitivity, such as the PVA/TiO2 [11], SnO2 − MoS2 [12], and α-Fe2O3@MoS2 [15]. The adsorption performance of MoS2 is inevitably limited due to the inadequate layer spacing, interlayer stacking and agglomerate [32]. Studies had shown doping could significantly improve the adsorption performance of MoS2 [33]. Hydrogel with thiourea group is widely used Cd2 + sensing material. Firstly, the Cd2+ can specifically bind to the thiourea group, forming a complex. Secondly, the “-S-Cd-S-” cross-linking structure will increase the cross-linking density of the hydrogel and then cause volume shrinkage. Finally, such coordination between Cd2+ and thiourea group will reduce the total charges, thus reducing the osmotic pressure, also leading to the volume shrinkage of the hydrogel. All above makes the hydrogel with thiourea group become a promising Cd2+ sensing material.
5. Conclusion
A highly sensitive Cd2+ ions sensor was proposed by using the hydrogel coated Ex-TFG. The transmission spectra of the Ex-TFG were investigated both experimentally and numerically. Hydrogel with allyl thiourea functional monomer makes the sensing membrane specifically bind to Cd2+, and forming cross linked ‘S-Cd-S’ structure. Benefitting by the strong interaction between the high order cladding mode of Ex-TFG and the hydrogel with tight linked Cd2+, the Cd2+ sensitivity is as high as 10600 nm/µM, and the experimental LOD is 20 pM (around 0.004 ppb), which is much less than that (3 ppb) of the international standard. In conclusion, The Ex-TFG coated with hydrogel holds the advantages of high sensitivity, ultra-low detection limit, high selectivity, good robust, and compact structure.
Funding
National Natural Science Foundation of China (21676034, 61975022).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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